This must be about the commonest conversation in any doctor's surgery.

Doctor: If you keep on smoking, you'll end up with a terrible chest/heart attack/stroke.

Patient: But my grandfather/uncle/father smoked like a chimney, lived on till he was 97 and finally died by falling under a bus!

That's top of our "heartsink" list. Because we know we won't get anywhere with the patient. And that's sad, because for everyone like grandad there are hundreds who die early from smoking-related diseases - and smokers conveniently forget about them.

But this isn't another tirade against smoking. It's actually about the smoker who gets away with it...

I'm just back from two medical conferences, one on lung disease and one on blood vessel problems, and I was struck by how much we are learning about why we don't become ill. Much of it is down to what we inherit. Smoking is a good example.

Around one in three of all smokers develop (and most eventually die from) chronic bronchitis - it's now called chronic obstructive pulmonary disease or COPD for short. The researchers have found that whether we get COPD or not depends on the chest infections we have had as children, and how we have reacted to them.

The main culprit is a family of viruses called adenoviruses. All they may do is cause a cough or flu-like illness. It may even be so mild that the family don't pay any heed. But the virus then lies around in the lungs causing no symptoms at all - until the child grows up and starts to smoke.

The combination of the tars and gases in the smoke and those persistent virus particles causes a severe reaction in the deepest part of the lungs - the smallest airways. They redden and swell, their walls thicken and they produce more mucus than usual. Yet they are so insensitive to pain and inflammation that the person knows nothing about it.

Until he/she gets a smoker's cough, and starts to get a bit breathless running after a bus, or going upstairs. By that time, more than half the lung is affected by the inflammation.

The researchers know exactly the type of inflammation that leads to COPD - the types of white cells involved (they are quite different from those that produce asthma) and the chemicals they produce that do the damage. And they know that if people stop smoking at this stage, the lungs can revert back absolutely to normal. It's harder, though, if it is allowed to go on for many more years. The more cigarettes smoked per day, for more years, the harder it is for the lungs to heal.

So why did grandfather not die from his habit? The current theory is that he was one of the lucky ones - he didn't get an adenovirus infection as a child! Or he inherited an immune system that didn't set up this very precise form of inflammation in his lungs in response to the smoke. The same goes for the fat man or woman who survives into old age, without a heart attack or stroke. All conventional argument says the amounts of fat in their bloodstreams should have given them a heart attack, heart failure or a stroke in their mid-fifties. This was explained by the speakers at my second conference.

The secret lies in the lining of their arteries. The inner lining of every blood vessel in the body is an incredibly thin layer of cells called the endothelium. It's not just a barrier between the blood and the tissues - it's a very active organ, controlling not only how wide the vessel should be (it has to open and close to regulate the blood pressure and shift blood between one part of the body and the other) but also what substances pass across it between the blood and the tissues.

This year's Nobel Prize in Medicine was won by the three men who discovered how the endothelium works. The secret of living a long and happy life while a bit overweight is all down to a healthy endothelium. There are some people whose endothelium knows exactly what to do with fat without storing it in blood vessel walls - and that, too, depends a lot on the sort of immune system you have inherited.

Fats set up the same sort of inflammation in blood vessel walls as smoking sets up in lungs - with similar disastrous results. And it looks suspiciously as if the reason some people get fat deposits in their arteries is that they had an infection many years before - which they didn't know about.

The problem is that smokers or overweight people can't yet find out if they are at high or low risk. But be sure that the researchers are working on it! In the meantime it's best not to smoke and to stick to a reasonable weight for your height.

Converted for the new archive on 30 June 2000. Some images and formatting may have been lost in the conversion.